// Copyright 2012 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include   "base/threading/platform_thread.h"
#include  "base/synchronization/once.h"



namespace base {

void CallOnceImpl(OnceType* once, PointerArgFunction init_func, void* arg) {
  base::subtle::AtomicWord state = base::subtle::Acquire_Load(once);
  // Fast path. The provided function was already executed.
  if (state == ONCE_STATE_DONE) {
    return;
  }

  // The function execution did not complete yet. The once object can be in one
  // of the two following states:
  //   - UNINITIALIZED: We are the first thread calling this function.
  //   - EXECUTING_FUNCTION: Another thread is already executing the function.
  //
  // First, try to change the state from UNINITIALIZED to EXECUTING_FUNCTION
  // atomically.
  state = base::subtle::Acquire_CompareAndSwap(
      once, ONCE_STATE_UNINITIALIZED, ONCE_STATE_EXECUTING_FUNCTION);
  if (state == ONCE_STATE_UNINITIALIZED) {
    // We are the first thread to call this function, so we have to call the
    // function.
    init_func(arg);
    base::subtle::Release_Store(once, ONCE_STATE_DONE);
  } else {
    // Another thread has already started executing the function. We need to
    // wait until it completes the initialization.
    while (state == ONCE_STATE_EXECUTING_FUNCTION) {
      base::PlatformThread::YieldCurrentThread();
      state = base::subtle::Acquire_Load(once);
    }
  }
}

} 
